Considerable growth has occurred in the field of structural liquid composite molding which in the art is often called liquid composite molding or structural reaction injection molding. For purposes of the present application all of these processes will be referred to for simplicity as structural liquid composite molding. In the manufacture of molded parts by structural liquid composite molding two basic processes in regard to the preform reinforcement production are employed. One involves the use of thermoformable continuous strand mats and the other process involves the utilization of multi-end roving in chopped form. The invention of this application is directed to the utilization of glass fiber rovings in chopped form and the formation of a preform on a preform screen from these rovings either above or mixed with other strands or rovings such as continuous strands or rovings.
In one process employed today to prepare preforms for use in structural liquid composite molding to prepare the fiber reinforcement, choppers are used. The choppers fracture, break or sever the glass fiber rovings used into discreet bundles which are transported out of the chopper with an air blower. As the chopped roving leaves the chopping machine, binder is applied to them. The chopper is normally connected to a hose-like mechanism that is used to convey the chopped glass in the air from the blower toward the preform screen. The chopped fibers are sprayed with binder as they leave the hose and before they reach the perforated metal screen. The preform screen is shaped in a form identical to the part that is to be molded. When the requisite amount of resin impregnated glass is placed on the screen, it is cured on the screen. After curing, the resin impregnated glass, now a preform, is ready for removal from the screen and for use in a mold. The preform part so made is placed in the mold and the mold is then closed. A low viscosity resin is then injected into the mold, and typically the mold is heated to provide the requisite heat. The heat is applied to the mold for a time sufficient to accomplish curing of the resin injected therein. After curing, the molded part is removed for use.
The typical preform screen utilized to prepare the preforms of the instant invention is mounted on a generally flat, solid metal frame section. The perforated section or screen is surrounded by the flat, solid metal transition frame. The screens normally are shaped to conform to the contour of the part that is to be molded, and in the transition area, i.e., the area between the screen portion and its attachment to the machine on which it is employed, there is a flat solid metal area or frame which is used to mount the screen to the preform machine. These screens have been found in the past to produce satisfactory parts, but there is a problem with excessive trim. The glass fibers in the transition area from screen to flat metal frame accumulate on the walls of the frame and stick thereto. The resulting preform, as a result, typically has peripheral edges which are at best ragged, i.e., fibers protrude from the dense portion of the periphery outwardly and prior to molding frequently must be trimmed. It has also been found in the transition area, i.e., the area from the screen to the flat metal frame where the screen is mounted on the preform machine, that there is a tendency for the part to contain a gradual transition in density of the fibers from the screen to the flat transition area. This results in edges on preforms produced by these screens which are sometimes weak or inadequate in strength at the edge of the molded part.